Chemical transformations of mono- and bis(buta-1,3-dien-1-yl)porphyrins:: A new synthetic approach to mono- and dibenzoporphyrins

beta-Butadienyl- and beta,beta'-dibutadienylporphyrins were prepared by the Wittig reaction of beta-formyl- and beta,beta'-diformylmeso-tetraphenylporphyrins with allylic phosphorus ylide. Subsequent treatment of beta-butadienylporphyrin with dienophiles afforded the corresponding Diels-Alder adducts. In the absence of dienophiles, beta-butadienylporphyrin underwent electrocyclization, followed by oxidation, to give monobenzoporphyrin in good yield. Similarly, adjacent and opposite dibenzoporphyrins were successfully synthesized from adjacent and opposite beta,beta'-dibutadienylporphyrins, respectively. This is the first report of electrocyclization of beta-butadienylporphyrins. The structures of mono- and dibenzoporphyrin nickel complexes, as well as of a Diels-Alder adduct, were determined by single-crystal X-ray diffraction; a strong distortion from the planarity of the porphyrin core was observed. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008.